An interesting question. 🙄 Here's what I think... As long as you do not destroy or otherwise change the partitioning of your hard drive, re-formatting the OS X partition and re-installing Leopard should not destroy or remove the Windows partition.
However, since Murphy's Law is more powerful than our human minds, make a backup of your Windows installation first using "WinClone". It's free. 😀
Pop in your leopard disk and follow the install though, you can chose which partition to reinstall leopard on during the install it wont effect the windows partition in anyway